Nickel-aluminide (NbAl) is an ordered intermetallic compound having unique properties. The strength of nickel-aluminide increases with temperature and it is stable over a wide range of chemical composition. The tendency of Ni-Aluminide for long range ordering restricts atomic mobility in an alloy. It is therefore resistant to diffusion controlled process which helps retain high temperature strength for a long time. In addition to stable high temperature strength, nickel-aluminide forms adherent aluminum oxide scale which enhances its resistance to high temperature oxidation and corrosion. All these salient features have promoted nickel-aluminide as a viable candidate for high temperature furnace application. While single crystal nickel-aluminide has excellent high temperature properties, the poly crystalline material has grain boundary ductility at high temperature, which makes it brittle during manufacturing of an engineered component due to generation of cracks along grain boundaries.
Early in 2000, Oak Ridge National Laboratories (ORNL) conducted extensive research in which it developed a cast Ni-Aluminide composition for industrial applications and patented the alloy design. The first industrial application chosen was high temperature furnace roll. A typical a furnace roll has two main components: 1) a cylindrical sleeve (hollow or solid) and 2) bells (bell shaped domes joined at either end of the sleeve). The bell and sleeve can be of similar material or different. During service the sleeve will endure more service load and be exposed to higher temperatures than the bells. Therefore to save cost, the bells can be made of lower grade material. ORNL selected More-1 as the alloy for the bells and nickel-aluminide as the sleeve material. During the fabrication process ORNL discovered that the developed nickel-aluminide had a tendency to generate cracking in the heat affected zone (HAZ) of the welds. To mitigate cracking 1.8% Zr was added to original nickel-aluminide composition. At the same time, to avoid cracking due to dilution a matching filler alloy was invented (IC221 LA filler wire or rod), Thus ORNL designed and developed a cast nickel-aluminide composition and filler wire for furnace roll application. These prior art rolls were implemented at the ArcelorMittal Burns Harbor quench and temper (Q&T) furnace.
In 2005, it was found that the installed rolls internally oxidized and developed nickel oxide protrusions on the roll surfaces. These nickel oxide protrusions have been causing surface defects during rolling of furnace reheated steel. Research at ArcelorMittal Global Research found that Zirconium (Zr) causes grain boundary oxidation and promotes surface oxidation causing the protrusions. To avoid this internal oxidation and surface oxide protrusions a furnace roll chemical composition free from Zr was developed. However, the published literature has emphasized that the presence of Zr at grain boundaries is required to improve the weldability of the nickel-aluminide and that without Zr in the roll, grain boundary cracking might occur.
Thus, there is needed a viable welding procedure to join cast low-Zr or Zr-free nickel-aluminide to itself or other nickel-base material.